Muzzle loading rifles have an essentially closed breech at the rear of the barrel, so that powder and bullets must be loaded at the muzzle or forward end of the barrel. A typical muzzle loading rifle has a barrel with a breech plug attached to occupy an enlarged rear bore portion of the barrel at the breech end. In some rifles, the breech plug is permanently attached. In others, the breech plug is removable to facilitate pass-through cleaning of the bore.
One type of removable breech plug plus is one that centers in the bore with an O-ring, but which relies on the rear support of the standing breech of the break-open action to retain it against the forces of discharge.
A safer existing removable breech plug employs a finely threaded body that screws into the rear of the barrel, with 10-15 turns to secure it in place. This provides safety against hang fires and facilitates removal for cleaning. However, the number of turns requires significant undesirable time and effort to remove and replace the plug. Moreover, the fouling associated with muzzle loading rifles can clog the threads, requiring undesirably great torque to remove the plug, through the many rotations required.
In addition, existing muzzle loading rifle breech plugs that use shotgun style primers do not fully support the primer used for firing. Such breech plugs have a rear face defining a central bore that receives a primer. The primer is a cylinder with a rear flange. However, because an extractor lip needs to be positioned between the primer flange and the breech plug's rear face, the flange of the primer is spaced apart from the breech plug's rear face by the thickness of the extractor lip. This portion is unsupported, and presents a risk of rupture when firing, allowing unwanted fouling to occur.
Because unloading a muzzle loading rifle via the muzzle is difficult or time consuming without discharging the rifle, and requires special tools, some shooters may be tempted to leave a loaded rifle in an unsafe charged condition. Conventional removable breach plugs may be used to unload via the breech, however these have the speed and convenience disadvantages noted above.